PiRelay, a Raspberry Pi relay shield with a mobile app that costs just US$17.06
There are many Raspberry Pi Relay HATs on the market, so the PiRelay by SB-Components should not necessarily turn heads. Its ability to control up to 4 devices with its 4 relays distinguishes it from most Relay HATs, which often come with 3 relays instead. These could be used for connecting the Pi Relay to lights, security systems or practically anything you could wire to the board.
If that has not caught your interest, then perhaps its purpose-built mobile app will. The PiRelay app is currently only available to download from the Google Play Store, where it has an average rating of 4.4 stars from 89 reviews. The app can control up to 100 relays connected to between 1 and 10 Raspberry Pis. The free to download app contains adverts, although you can purchase the ad-free version for £4.99 (~US$6.35) if you find it useful.
The PiRelay HAT comes with 4 Chipskey HLS8L-DC5V-S-C relays that can handle up to 240 VAC/7 A, or 125 VDC/10 A. SB-Components has integrated a LED status light for each relay too, along with a 40-pin stacking header for accessing a Raspberry Pi's GPIO. You can connect the PiRelay without GPIO though, the method by which SB-Components lists on its product page.
The company has published what looks like an extensive wiki on GitHub with a link to install the piCube software needed to get the PiRelay working. The PiRelay costs US$17.06 on SB-Components, which ships worldwide. The company accepts Amazon Pay, Google Play and PayPal should not feel comfortable transacting directly through its website.